There are two well established and different techniques for guiding aircraft to a safe landing in inclement weather. A first technique consists of radiating signals from ground based directional antennas with such signals being received in the aircraft and processed accordingly to provide aircraft guidance to a landing. The airborne processor attached to the receiver operates on the received signals in accordance with knowledge of the nature of the guidance beams radiated. This first technique can be classified as an "air derived" system, since the basic guidance data is derived in the aircraft. ILS, the newer MLS and the system described in my U.S. Pat. No. 4,429,312 are air derived systems.
A second technique uses a ground based radar that locates the aircraft via conventional radar practices, (i.e., radiating a transmission and determining the aircraft location via ground based reception) using directional antenna(s), and the aircraft skin echo resulting from that transmission. This second technique can be classified as a "ground derived" system, since the basic aircraft location is derived on the ground and relayed to the aircraft, via voice, for example. GCA (Ground Controlled Approach), also called PAR (Precision Approach Radar), is an example of such a ground derived system. A basic advantage of GCA, is that it can be used to recover minimally equipped aircraft in that all that is required in the aircraft to obtain guidance data is a voice radio that the pilot can use to receive the ground derived guidance data. GCA is thus highly desirable for military purposes and currently finds its use in such applications.
The nature of the guidance technique employed in air derived systems can be further classified into the use of fixed beams and scanning beams. An example of the use of fixed beams is provided by ILS and by the system described in my U.S. Pat. No. 4,429,312. In ILS, paired overlapping fixed beams define a localizer, or azimuth guidance path, and additional paired, overlapping fixed beams define a glideslope path. When the aircraft receiver and associated guidance processor separately indicate that the localizer and glideslope paired, overlapping fixed guidance beam signals are of equal intensity, the aircraft is on the desired landing approach path. ILS is characterized by providing only one desired, prescribed, or predetermined landing path, generally runway centerline and one fixed glideslope. It is further characterized by relatively simply ground based equipment employing an array of fixed beam antennas.
An example of the use of scanning beams in an air derived system is provided by the relative new FAA approved MLS. In MLS, a narrow precision guidance beam is scanned in azimuth, about the runway centerline. Additionally, a separate beam is scanned in elevation. In the aircraft, a receiver and associated processor detects the passage of such scanning beams and, together with knowledge of the nature of the scanning process, determines the aircraft location with respect to a desired landing path. MLS is characterized by providing pilot selectable approach paths. MLS is also characterized by highly complex ground equipment, required for the generation of the scanning beams.
GCA provides an example of the use of scanning beams in a ground derived system. GCA employs separate azimuth and elevation narrow scanning beams for aircraft location and associated recovery purposes. These beams scan the desired approach region in azimuth, and in elevation, and determine aircraft location with respect to a desired azimuth and elevation approach path by virtue of the range and azimuth (or elevation) at which the aircraft skin echo is detected in such azimuth and elevation scanning processes.
ILS, an air derived fixed beam guidance system, evolved prior to the development of the scanning beam GCA system. The development of scanning beam MLS was initiated in the 60's, long after both GCA and ILS were operational, as an ILS replacement system that would ostensibly overcome the then existing deficiencies of the fixed beam low frequency ILS system. In this MLS development program, scanning beams, as opposed to fixed beam guidance technology, were employed in order to provide the pilot flexibility in selecting a landing guidance path.
Currently, the MLS program faces serious user acceptance to its being implemented as an ILS replacement system. This objection is primarily based on the fact that the fixed beam ILS system, with the improvements that have been made in it since the 60's, provides very acceptable landing guidance.
The military, however, still has a need for a GCA type of system for operating in the battle environment with minimally configured aircraft, both manned and unmanned (drones or RPV's). The complex scanning beam mechanism of current GCA equipment, with its associated initial cost and required maintenance, poses a serious drawback to its continued use however and, hence, the military is tending to transition to air derived systems such as ILS, and the microwave fixed beam landing system described in my U.S. Pat. No. 4,429,312, and MLS, if MLS ground equipment can be made sufficiently light and compact, a task that has yet to be accomplished, despite expenditure of considerable time and effort.
What is required then is a ground derived aircraft recovery system without the complexity and cost of the current GCA scanning beam system.